The invention relates to an actuator with a simple function for the control of a friction clutch in the drive train of a motor vehicle having a cylinder/piston unit for generating the contact pressure of the clutch, in conjunction with which a controllable electric motor/pump unit provides a pressure medium and is connected to the cylinder/piston unit via a self-regulating valve unit, so that the clutch is controlled by actuating the pump. The invention can thus relate both to clutches for locking a differential and to clutches for controlling the torque directed to an axle or to a wheel. The actuator acts in the closing direction of the clutch, and an internal force inside the clutch, such as a spring, acts in the opening direction.
The requirements in respect of the controllability of friction clutches are very high for applications in a motor vehicle, both with regard to the accuracy of adjustment of a specific torque and with regard to the speed of the control. The latter is particularly applicable to the releasing of the clutch, for example in the case of operation in conjunction with ABS or ESB. The need for inherent safety is also relevant in addition. This means that the safest condition (most often, this will be the clutch in its released state) must be adopted in the event of a system failure.
According to the general state of the art, the contact pressure necessary for clutches of the kind in question is applied either mechanically or hydraulically. Ramp rings, for example, find an application in the former case, for the rotation of which a source of electrical power is used. Mechanisms of this kind are disadvantageous, if only because of the large number of components with their associated friction and free play.
In the case of hydraulic actuation by means of an external pump, the speed of response required for interaction with an electronic system can only be achieved with a heavily dimensioned combined electric motor and pump unit, which is out of the question for reasons of weight and, above all, power consumption. A pressure accumulator and an actuated control valve are also required. If a pump that is dependent on the difference in the speed of rotation and rotates together with the clutch components is used instead of these, the drive must be transmitted to a rotating part, which is costly and disadvantageous. A further problem is that, for a low difference in speed, the pressure necessary for control is not available.
In the case of rather slower operation, and with a specific torque value to be transmitted, moreover, the pressure of the pressure medium acting on the clutch must be maintained, which means high circulation losses for a non-controllable electric motor/pump unit. In all previously disclosed systems, both the release speed and the inherent safety leave something to be desired.
An actuator of the kind in question is previously disclosed in EP 348 270, in which control is achieved by means of a non-reversible motor/pump unit. The self-regulating valve unit contains, in addition to a dump valve, a differential slide, which keeps the dump valve closed only in the presence of an adequate flow rate for the fluid supplied by the pump. As a result, the energy consumption is considerable if the pressure in the cylinder/piston unit must be maintained.
Also proposed in EP 1 256 478, which has not previously been published, are a reversible, controllable pump and a dump valve, which valve responds to the difference between the pressure generated by the pump and the pressure prevailing in the cylinder/piston unit. In order to permit rapid opening of the clutch, a throttled secondary circuit is required, although this makes it impossible to maintain the pressure in the cylinder/piston unit in the presence of a low supply or in the absence of any supply.
The object of the invention is to provided an actuator which does not exhibit the aforementioned disadvantages, and which is accordingly fast, inherently safe, simple and inexpensive. The actuator permits the clutch to be maintained in the engaged position with a minimum supply from the pump.